Viola cazorlensis Gand. (Violaceae) is a
perennial, suffruticose violet endemic to a few limestone mountain ranges in
south-eastern Spain (see Proyecto Anthos).
This species, along with two closely related taxa occurring in the Balkan
Peninsula (V. delphinantha and V. kosaninii), constitute the small
section Delphiniopsis of the genus Viola, an old and apparently primitive
lineage. It occurs as small, discrete populations, almost invariably associated
with scattered rocky outcrops, cliffs, and pockets of sandy soils originating
from heavily weathered limestone on mountain ridges. Populations are largely
variable in size.

Our early studies focused on floral morphology and the
interaction with pollinators. The pinkish-purple flowers are characterized by a
long spur (up to 37 mm in length, mean = 25.1 mm), where nectar accumulates at
the distal portion (0.24 ± 0.28 ml per flower). They vary widely in morphology
within and among populations. Details on floral morphology variation can be
found
here. Flowers are self-compatible, although only about 1% of excluded
flowers are able to produce fruits autonomously, and the action of pollinators
increases dramatically fruit-set. The main pollinator is a day-flying hawk moth,
Macroglossum stellatarum (Lepidoptera, Sphingidae). The morphology of a
flower does not determine the probability of setting fruit, although peduncule
length largely influenced the number of seeds/fruit. Plant seed production did
however depend more of total flower production. Thus, floral variability in this
species may be explained by the combined action of selection on peduncle length
(the character most variable among individuals) and little stabilizing selection
on spur length and size of petals (see
here). This was one
of the first study cases in which directional selection of pollinators on
corolla or spur length was not verified (see also
here). Studies using a population genomic approach have shown that among-population variation in floral characteristics observed in this
species reflects adaptive divergence driven by geographically variable
selection (read more).
In addition, adaptive genetic divergence of populations is significantly
correlated with epigenetic differentiation, as shown by a study on patterns of
DNA methylation (read
more).

Although individual differences in fecundity are influenced
by extrinsic factors, such as microhabitat type and interactions with herbivores,
a sizeable fraction of individual differences in long-term maternal fecundity can
be explained by genetic differences alone (read
more). Type of substrate (rock cliffs, bare rocks at ground level, and sandy
soils) largely determines differences between V. cazorlensis plants in
the impact of herbivory. Plants growing on the soil are those incurring the
greatest reproductive losses to herbivores. Also cliff plants tend to be the
largest and rock plants the smallest. Plant size, in turn, influences the
supra-annual frequency of flowering and the number of flowers produced in each
reproductive event. Furthermore, the proportion of reproductive individuals
within a population decreases in years with low precipitation in January-March
(the three months previous to floral differentiation). As a consequence the
influence of floral morphology in individual differences in fecundity is
relatively small (see
here for more details).

In a long-term study (1988-2003) in a single population,
both the proportion of flowering individuals per year and the average number of
fruits per plant showed a negative temporal trend, indicating a progressive
reduction of the population reproductive success that was particularly strong in
plants growing in the soil. Among the potential causes of such pattern herbivory
and winter drought appeared as the strongest determinants of reproductive
decline. Consumption by ungulates showed the opposite temporal trend and both
the proportion of individuals affected and the individual percentage of biomass
consumed tended to increase in the second half of the study period, causing a
reduction in subsequent flower production. Again plants growing at ground level
were more negatively affected by herbivores. Winter (January – March)
precipitation was relatively low within the 16 yr study period as a consequence
of a progressive reduction of precipitation in those months observed in the
period 1955-2003. Reduced rainfall did also negatively affect fruit production
within the study period, thus contributing to the observed reduction in plant
reproduction success that could impose an increasing risk to the conservation of
this endemic violet.